The present invention relates generally to radar processing, and more particularly to the recording of echoes from high resolution radar in real time.
A radar system transmitting a radar signal will receive echo returns at different time intervals from a plurality of targets with the time intervals depending on the relative distance of each target from the radar transmission source. That is, a close target return will reach the source relatively sooner than a far target return. It is apparent that the returns from a plurality of targets all resulting from one radar transmission are time sequential and, by identifying each target with a time range, one target return may be discriminated from all other target returns.
In order to perform the association of each target with a particular time slot, the time receipt of the target range of interest may be divided into time bins/range-cells. Then each time-bin/range-cell may be separately interrogated to determine the characteristic of any targets that may be present.
It is a common practice in the prior art to utilize a clock controlled delay line such as, for example, a clock-controlled shift register, as a range buffer so that the storage bits of the shift register define range bins. Radar return signals arriving during a particular pulse repetition interval (PRI) are sampled and clocked into the shift register in time sequential order until the first pulse return has reached the end of the shift register. At that point, the shift register clock is stopped and the data stored in the shift register is transferred out in parallel. Near targets appear in the high numbered range bins while far away targets appear in the low numbered range bins. The parallel outputs from the shift register range bins are then processed to extract the desired information.
"Bucket Brigades" and charge coupled devices (CCD's) have been available for some time for use as clock controlled delay lines. These devices operate relatively effectively on narrow band echos from radars which utilize long transmission pulses. However, the switching time for these devices is inherently limited by the charging time constant .tau.=R.sub.e C, where R.sub.e is the internal resistance of the device and C is the capacitance of the individual FET's. In order to increase the switching speed of these devices, R.sub.e C must be made small. However, the ON and OFF resistance of the FET's is substantially set by the choice of FET material. Likewise, the capacitance value of the FET's is limited by the material chosen and by the size of the control electrodes used thereon. This limitation on the charge stepping time in the individual FET's accordingly limits the clock rate which may be utilized on the delay line.
In high range resolution radars, a wide bandwidth/short pulse is utilized to significantly reduced the size of the time bin/range cell according to the equation: ##EQU1## where C is the speed of light. These wide bandwidth/short pulses utilized in the high range resolution radars require significantly higher clock rates than may be utilized with "Bucket Brigades" and charge coupled devices. If the designer was to ignore the charging and discharging time limitations on these devices, and increase the clock frequency beyond that which the FET material can handle, then the FET would not charge properly during a single range cell or charging period but would instead leave charge in the FET capacitor which would be seen in the next range cell. Accordingly, this charge spread would result in the averaging of the signal over a number of range bins, resulting in a significant transfer loss, and a concommittant resolution loss. Thus, these low clock rates available for use with "Bucket Brigades" and charge coupled devices, in essence inherently limit these devices to narrow bandwidth systems because the clock rates set the range gate sizes.